Cassini Spacecraft Zooming Past Two Saturn Moons

By SPACE.com Staff |
May 19, 2010 04:44pm ET

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On the left, Saturn's moon Enceladus is backlit by the sun, showing the fountain-like sources of the fine spray of material that towers over the south polar region. On the right, is a composite image of Titan.

Credit: NASA/JPL/SSI and NASA/JPL/University of Arizona

NASA's Cassini spacecraft is whipping through a
double-header at Saturn to catch an up-close glimpse of two of the ringed
planet's moons ? the icy geyser-ridden Enceladus and the cloud-covered Titan.

The fortuitous alignment of the two Saturn
moons means that the Cassini probe will be able to observe these
contrasting worlds within less than 48 hours, with no maneuver in between.

Cassini
has already swooped by Enceladus, making its closest approach early on May 18 GMT
(late at night on May 17 Pacific Time). The spacecraft passed within about 270
miles (435 km) of the moon's surface.

The main goal of the Enceladus flyby was to watch the sun
behind the moon's water-rich
plume that flares out from its south polar region.

Scientists used Cassini's ultraviolet imaging spectrograph to
measure whether there is molecular nitrogen in the plume, which is already
known to contain ammonia. Heat can allow ammonia molecules to break into nitrogen
molecules and scientists want to know if that's happening at Enceladus.

By determining the nitrogen content of Enceladus' geyser
plume, scientists could unearth clues about thermal processing in the interior
of the icy moon.

Cassini's second flyby is taking it past Saturn's largest
moon, Titan.

The closest approach will take place in the early hours of May 20 GMT, or late
evening on May 19 Pacific time at NASA's Jet Propulsion Laboratory in Pasadena,
Calif., where engineers watch over Cassini. The spacecraft will glide to within
750 miles (1,400 km) of the surface of the moon.

Cassini will perform radio science experiments as it passes
Titan, in order to detect the subtle variations in the gravitational tug on the
spacecraft by the giant moon, which is 25 percent larger in volume than the
planet Mercury.

Scientists will analyze the data to get a clearer
understanding of Titan's internal structure, and to learn whether the moon has
a liquid ocean beneath its surface. The composite infrared spectrometer will
also use its southernmost pass to collect thermal data to fill out its temperature
map of the smoggy moon.

Cassini has made four previous
flybys, with one more planned for the years ahead.

The Cassini probe launched in 1997 and arrived at Saturn in
2004, where it dropped the European Huygens probe on the cloudy surface of
Titan. Cassini was slated to be decommissioned in September of this year, but
has received an extended mission that now runs through 2017.